Fix inferior deadlock with "target remote | CMD"

diff --git a/gdb/ser-base.c b/gdb/ser-base.c
index 790cb1b..6c895ef 100644
--- a/gdb/ser-base.c
+++ b/gdb/ser-base.c
@@ -288,6 +288,8 @@  ser_base_read_error_fd (struct serial *scb, int close_fd)
 	  if (s == 0 && close_fd)
 	    {
 	      /* End of file.  */
+	      if (serial_is_async_p (scb))
+		delete_file_handler (scb->error_fd);
 	      close (scb->error_fd);
 	      scb->error_fd = -1;
 	      break;
@@ -313,6 +315,17 @@  ser_base_read_error_fd (struct serial *scb, int close_fd)
     }
 }
 
+/* Event-loop callback for a serial's error_fd.  Flushes any error
+   output we might have.  */
+
+static void
+handle_error_fd (int error, gdb_client_data client_data)
+{
+  serial *scb = (serial *) client_data;
+
+  ser_base_read_error_fd (scb, 0);
+}
+
 /* Read a character with user-specified timeout.  TIMEOUT is number of
    seconds to wait, or -1 to wait forever.  Use timeout of 0 to effect
    a poll.  Returns char if successful.  Returns SERIAL_TIMEOUT if
@@ -589,6 +602,10 @@  ser_base_async (struct serial *scb,
 	fprintf_unfiltered (gdb_stdlog, "[fd%d->asynchronous]\n",
 			    scb->fd);
       reschedule (scb);
+
+      if (scb->error_fd != -1)
+	add_file_handler (scb->error_fd, handle_error_fd, scb);
+
     }
   else
     {
@@ -607,5 +624,8 @@  ser_base_async (struct serial *scb,
 	  delete_timer (scb->async_state);
 	  break;
 	}
+
+      if (scb->error_fd != -1)
+	delete_file_handler (scb->error_fd);
     }
 }
diff --git a/gdb/testsuite/gdb.base/long-inferior-output.c b/gdb/testsuite/gdb.base/long-inferior-output.c
new file mode 100644
index 0000000..eb3d5c5
--- /dev/null
+++ b/gdb/testsuite/gdb.base/long-inferior-output.c
@@ -0,0 +1,38 @@ 
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+
+static size_t total_bytes = 0;
+
+int
+main ()
+{
+  int i = 0;
+
+  /* When testing with "target remote |", stdout is a pipe, and thus
+     block buffered by default.  Force it to be line buffered.  */
+  setvbuf (stdout, NULL, _IOLBF, 0);
+
+  /* This outputs > 70 KB which is larger than the default pipe buffer
+     size on most systems (typically 16 KB or 64 KB).  */
+  for (i = 0; i < 3000; i++)
+    total_bytes += printf ("this is line number %d\n", i);
+
+  printf ("total bytes written = %u\n", (unsigned) total_bytes);
+  return 0; /* printing done */
+}
diff --git a/gdb/testsuite/gdb.base/long-inferior-output.exp b/gdb/testsuite/gdb.base/long-inferior-output.exp
new file mode 100644
index 0000000..ae9386c
--- /dev/null
+++ b/gdb/testsuite/gdb.base/long-inferior-output.exp
@@ -0,0 +1,75 @@ 
+# Copyright 2017 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# When debugging with "target remote |", the inferior's output is
+# connected to a pipe, and if GDB doesn't flush the pipe while the
+# inferior is running and the pipe becomes full, then the inferior
+# deadlocks:
+#
+#  1. User sets breakpoint, and types "continue"
+#
+#  2. Inferior prints to stdout/stderr before reaching breakpoint
+#     location.
+#
+#  3. The output pipe becomes full, so the inferior blocks forever in
+#     the printf/write call.
+#
+#  4. The breakpoint is never reached.
+
+if [target_info exists gdb,noinferiorio] {
+    verbose "Skipping because of noinferiorio."
+    return
+}
+
+standard_testfile
+
+if [prepare_for_testing "failed to prepare" $testfile {} {debug}] {
+    return -1
+}
+
+if { ![runto_main] } then {
+    fail "run to main"
+    return
+}
+
+set printing_done_line [gdb_get_line_number "printing done"]
+gdb_test "break $printing_done_line" ".*" "set breakpoint after printing"
+
+send_gdb "continue\n"
+
+set expected_lines 3000
+set more 1
+set i 0
+while {$more} {
+    set more 0
+    gdb_expect {
+	-i $inferior_spawn_id
+	-ex "this is line number $i" {
+	    incr i
+           if {$i != $expected_lines} {
+		set more 1
+	    }
+	}
+    }
+}
+
+gdb_assert {$i == $expected_lines} "saw all lines"
+
+set test "breakpoint reached"
+gdb_test_multiple "" $test {
+    -re "Breakpoint .* main .*$srcfile:$printing_done_line.*$gdb_prompt $" {
+	pass $test
+    }
+}